On Application of Large Eddy Simulations for Gas Turbine Combustion System

As one of the core components of gas turbines, the combustion system operates in a high-temperature and high-pressure adverse environment, which makes it extremely prone to faults and catastrophic accidents. Therefore, it is necessary to monitor the combustion system to detect in a timely way whether its performance has deteriorated, to improve the safety and economy of gas turbine operation. However, the combustor outlet temperature is so high that conventional sensors cannot work in such a harsh environment for a long time. In practical application, temperature thermocouples distributed at the turbine outlet are used to monitor the exhaust gas temperature (EGT) to indirectly monitor the performance of the combustion system, but, the EGT is not only affected by faults but also influenced by many interference factors, such as ambient conditions, operating conditions, rotation and mixing of uneven hot gas, performance degradation of compressor, etc., which will reduce the sensitivity and reliability of fault detection. For this reason, many scholars have devoted themselves to the research of combustion system fault detection and proposed many excellent methods. However, few studies have compared these methods. This paper will introduce the main methods of combustion system fault detection and select current mainstream methods for analysis. And a circumferential temperature distribution model of gas turbine is established to simulate the EGT profile when a fault is coupled with interference factors, then use the simulation data to compare the detection results of selected methods. Besides, the comparison results are verified by the actual operation data of a gas turbine. Finally, through comparative research and mechanism analysis, the study points out a more suitable method for gas turbine combustion system fault detection and proposes possible development directions.

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High Resolution Large Eddy Simulations to Evaluate Turbulence Properties Within a Real Helicopter Engine Combustor

Volume 4A: Combustion, Fuels, and Emissions ◽

10.1115/gt2018-75131 ◽

2018 ◽

Author(s):

Charlie Koupper ◽

Jean Lamouroux ◽

Stephane Richard ◽

Gabriel Staffelbach

Keyword(s):

Gas Turbine ◽

Turbulence Intensity ◽

Numerical Scheme ◽

Mesh Refinement ◽

Large Eddy Simulations ◽

Scale Model ◽

Large Eddy ◽

Set Up ◽

High Level ◽

The Impact

In a gas turbine, the combustor is feeding the turbine with hot gases at a high level of turbulence which in turns strongly enhances the heat transfer in the turbine. It is thus of primary importance to properly characterize the turbulence properties found at the exit of a combustor to design the turbine at its real thermal constraint. This being said, real engine measurements of turbulence are extremely rare if not inexistent because of the harsh environment and difficulty to implement experimental techniques that usually operate at isothermal conditions (e.g. hot wire anemometry). As a counterpart, high fidelity unsteady numerical simulations using Large Eddy Simulations (LES) are now mature enough to simulate combustion processes and turbulence within gas turbine combustors. It is thus proposed here to assess the LES methodology to qualify turbulence within a real helicopter engine combustor operating at take-off conditions. In LES, the development of turbulence is primarily driven by the level of real viscosity in the calculation, which is the sum of three contributions: laminar (temperature linked), turbulent (generated by the sub-grid scale model) and artificial (numerics dependent). In this study, the impact of the two main sources of un-desired viscosity is investigated: the mesh refinement and numerical scheme. To do so, three grids containing 11, 33 and 220 million cells for a periodic sector of the combustor are tested as well as centred second (Lax-Wendroff) and third order (TTGC) in space schemes. The turbulence properties (intensity and integral scales) are evaluated based on highly sampled instantaneous solutions and compared between the available simulations. Results show first that the duration of the simulation is important to properly capture the level of turbulence. If short simulations (a few combustor through-times) may be sufficient to evaluate the turbulence intensity, a bias up to 14% is introduced for the turbulence length scales. In terms of calculation set-up, the mesh refinement is found to have a limited influence on the turbulence properties. The numerical scheme influence on the quantities studied here is small, highlighting that the employed schemes dissipation properties are already sufficient for turbulence characterization. Finally, spatially averaged values of turbulence intensity and lengthscale at the combustor exit are almost identically predicted in all cases. However, significant variations from hub to tip are reported, which questions the pertinence to use 0-D turbulence boundary conditions for turbines. Based on the set of simulations discussed in the paper, guidelines can be derived to adequately set-up (mesh, scheme) and run (duration, acquisition frequency) a LES when turbulence evaluation is concerned. As no experimental counterpart to this study is available, the conclusions mainly aim at knowing the possible numerical bias rather than commenting on the predictivity of the approach.

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